Industrial water-carrying vessels, such as process chests, pipes, process water storage tanks, additive tanks, filters, water supply pipes or waste-water pipes, etc., are often observed to have a growth coating one or more surfaces of the water-carrying vessel where the surfaces contact the water. This growth is actually a biofilm, a collection of microorganisms embedded in a matrix of extracellular polymeric substances and various organic and inorganic compounds. In the last several years, the nature of these biofilms has been the focus of attention among both academic and industrial researchers.
Although biofilms may contain a single species of microorganism, typically biofilms comprise not only different species of microorganisms but different types of microorganisms, for example algae, protozoa, bacteria and others. It has been found that one of the characterizing features of biofilms is that the microorganisms therein act cooperatively or synergistically. Thus, for example, the activity of certain enzymes produced by bacteria which are attached to a surface is observed to be much higher than the corresponding activity of the same enzymes produced by these bacteria in planktonic form, i.e. when free-floating (David G. Davies, in “Microbial Extracellular Polymeric Substances”, Springer-Verlag 1999; Editors: J. Wingender, T. R. New, H. C. Flemming, hereinafter “Wingender et al.”). Comparative studies of enzyme activities in planktonic bacteria and bacteria attached to solid surfaces which contact water have shown that enzymatic activity in attached bacteria is greater than in planktonic bacteria (M. Hoffman and Alan W. Decho in Wingender et al.). Communication within microbial biofilms is responsible for the induction and regulation of the activities of the biofilm, including for example extracellular enzyme biosynthesis, biofilm development, antibiotic biosynthesis biosurfactant production, exo-polysaccharide synthesis and more, all of which involve complex biochemical activity (Alan W. Decho in Wingender et al.). Exchange of genetic material between the microorganisms in biofilms has also been observed. Empirically it has been found that, in a given industrial water environment, microorganisms living in a biofilm are better protected from biocides than microorganisms living outside a biofilm. Thus, collectively the microorganisms embedded in a biofilm display characteristics which are different from the characteristics which are displayed by a like number of planktonic microorganisms.
By acting cooperatively, a collection of microorganisms acts as a microbial community: it is able to construct a matrix formed of inorganic and organic material and thus to form and maintain a biofilm. Since microorganisms are single-celled organisms that grow and multiply, the microorganisms in a biofilm must continually replenish the matrix around them, expand the matrix and maintain the matrix. This process can be likened to a group of people who act together to construct a contiguous set of dwelling units for themselves, and who then not only maintain the existing homes but also add additional homes to accommodate population growth, either by building contiguously horizontally or by adding new homes vertically on top of existing homes.
As scientists best understand it at present, the cooperative behavior between the microorganisms in biofilms is induced by communication between the microorganisms. For example, homoserine lactones play an important role in communication between bacteria. The extracellular polymer matrix of a biofilm seems to present an efficient medium for chemical communication and thus to promote more efficient communication between individual microorganisms embedded in the biofilm.
Because microorganisms in biofilms are more effective than planktonic microorganisms in producing enzymes, much interest has been shown in developing biofilms for effecting chemical reactions. However, in the context of industrial and process water-carrying vessels, such as conduits, water tanks and the like, this propensity to produce enzymes, and more importantly the tendency of biofilms to form heavy biomass on the surface of the vessel, can be extremely detrimental. As a biofilm grows, it may reduce the effective diameter of a pipe or other conduit at a particular point along the path of the water or increase friction along the flow path in the conduit, thus increasing resistance to the flow of water through the conduit, reducing the flow of water therethrough, increasing power consumption in the pumps which push or pull the water through the conduit, and decreasing the efficiency of industrial operations.
Biofilms also deteriorate the quality of various chemicals and process additives. For example, in the paper industry, biofilms cause deterioration of chemicals like starch and calcium carbonate slurries which are added to the pulp slurries in the wet end processes (K. Jokinen in “Papermaking Chemistry”, Part 4, 1999, Ed. Fapet Oy). Microorganisms are also responsible for hydrogen peroxide degradation in bleaching and de-inking systems (J. F. Kramer, MP Chemical Treatment, August 1997, pp. 42–50). The presence of H2O2-degrading enzymes in de-inkling and bleaching mills thus necessitates the feeding of larger quantities of hydrogen peroxide than would otherwise be necessary in order to meet the set point bleaching criteria, thereby increasing production costs.
Biofilms may also cause severe corrosion of pipes and chests, may cause severe problems in paper and board machines, and inter alia may cause deterioration of the quality of finished paper, foul odors and severe runnability problems.
Various methods have been described in the prior art in order to control biofilms in industry. One approach has been to physically destroy the biofilm by mechanical means, e.g. by scraping or by sonication. For example, U.S. Pat. No. 4,419,248 to Costerson describes a method for removing biofilm from a surface submerged in water. The method includes cooling the surface to below the freezing point of water to thereby generate large, sharp-edged ice crystals in the biofilm. The frozen biofilm is then thawed and removed from the surface by, for instance, flowing a liquid across the surface. This approach is often impractical, however, since the place where the biofilm grows may be inaccessible, and/or disruption of industrial operations may be required in order to reach the biofilm.
Another approach has been to physically destroy the biofilm by chemical means, e.g. by use of surface-active agents and detergents which cause the biofilm matrix to break apart. For example, U.S. Pat. No. 5,753,180 to Burger describes a non-biocidal method for inhibiting microbially influenced corrosion of susceptible metal surfaces having an anaerobic biofilm containing active sulfate-reducing bacteria, comprising contacting the biofilm with a liquid dispersion of an anthraquinone compound. U.S. Pat. No. 6,149,822 to Fabri describes a process for both removing and controlling biofilms present in industrial cooling and process waters. The process provides a composition which includes the reaction products of an amino base, formaldehyde, an alkylenepolyamine, and the ammonium salt of an inorganic or organic acid. The composition may be used to remove existing biofilms from process water equipment. Further lower maintenance dosages may be used to maintain the equipment in a substantially biofilm free condition. U.S. Pat. No. 5,670,055 to Yu et al. describes a method for dispersing biofilms in industrial process water, which comprises adding an effective biofilm dispersing amount of linear alkylbenzene sulfonate to industrial process water which contains slime-forming bacteria and other microorganisms. An alternative embodiment of the invention of Yu et al. comprises adding a compound selected from the group of biocides cited therein, combined with a biofilm dispersing agent from a list cited therein as well. U.S. Pat. No. 5,882,916 to Wiersma describes a decontamination process for reducing the surface tension of a biofilm, allowing for the removal of biofilm and the control of underlying bacteria. In accordance with the invention of Wiersma, a solution consisting of saponin and soft acid such as food grade sodium lactate is contacted with the biofilm. The saponin acts as a foaming agent, providing surface tension reduction capable of loosening the biofilm.
Approaches are known in the art in which the biofilm matrix is degraded by enzymes which are fed externally. For example, U.S. Pat. No. 6,100,080 to Johansen describes a method for cleaning and disinfecting a surface at least partly covered by a biofilm layer, comprising the steps of contacting the biofilm with a cleaning composition comprising one or more hydrolases, for either fully or partly removing or releasing the biofilm layer from the surface; and contacting the biofilm with a bactericidal disinfecting composition which comprises an oxidoreductase in an amount effective for killing the living bacterial cells present in the biofilm. Attack with external enzymes leads to loss of activity and changes in the properties of the biofilm. Such approaches preclude the ability of the microorganisms to maintain or expand the matrix. However, such approaches suffer from various drawbacks, for example the treatment may be too specific and results may vary in different sites, or the treatment may not be cost-effective.
An additional difficulty encountered in controlling biofilms in accordance with the prior art is that as the biofilm matrix decomposes, viable cells are usually released into the water. Such viable cells may start a new biofilm. Similarly, decomposition of the biofilm matrix may lead to release of enzymes into the water, which may affect the industrial processes being carried out.
In this regard, biocides can be useful. The use of biocides to treat planktonic bacteria in industrial process waters is known in the art. See, for example, the inventor's own U.S. Pat. Nos. 5,976,386 and 6,132,628, the contents of which are incorporated herein by reference, or U.S. Pat. No. 5,882,526 to Brown et al., which describes a method for treating regulated waters using a combination of a halogen-containing oxidizer, an erosion control agent, hydrogen peroxide, and a hydrogen peroxide stabilizer. More recently, biocides have been used in attempts to control biofilms. This goal has sometimes been achieved by combining a biofilm-degrading technique, such as feeding of biofilm-degrading enzymes or physical removal of biofilms, with the application of a biocide which enables the maintenance of a low count of planktonic microorganisms in the process water. For example, U.S. Pat. No. 5,789,239 to Eyers et. al. describes the use of (a) at least one enzyme from a defined group to degrade the biofilm and (b) a short-chain glycol as a biocide for the avoidance and/or removal of biofilm on surfaces. U.S. Pat. No. 4,966,716 to Favstritsky et al. describes a method for controlling the growth of microorganisms which reduce the efficiency of recirculating water systems comprising introducing into such systems a biocidally effective amount of a water soluble perhalide. The perhalide is first introduced in amounts sufficient to kill the microorganisms at film forming surfaces of the system. Thereafter, the concentration of organic ammonium perhalide is maintained at a level sufficient to reduce substantially the regrowth of such microorganisms.
Alternatively, biocides have been used to control microorganisms embedded in biofilms, i.e. to eradicate the microorganisms themselves within the biofilm matrix. Specifically, monochloroamines (MCAs) and free chlorine (FC) were claimed to show similar efficacy in disinfecting biofilm bacteria (M. W. LeChevallier et al., Applied and Environmental Microbiology, pp. 2492–2499, 1988; T. S. Rao et al., Biofouling 12(4) pp. 321–332, 1998). The difficulty with this approach, as stated above, is that empirically it has been found that eradicating microorganisms in biofilms requires concentrations of biocides which are several times higher than the concentrations of biocides required to eradicate planktonic microorganisms that long contact times between the biofilm microorganisms and the biocide are required, or that continuous application of the biocides is required. This increases the cost of treatment, and may expose workers to greater risks from the biocides than is desirable or allowable. It also poses a greater risk to the environment.
Approaches to biofilm control utilizing combinations of the above methods are also known in the art. These combination approaches, which are designed in an attempt to solve problems which emerge during the implementation of each approach separately, may also suffer from some of the drawbacks described above. For example, U.S. Pat. No. 6,106,854 to Belfer describes an aseptic disinfectant composition in liquid form having germicidal and biofilm cleaning properties comprising an anti-infective, an antiseptic agent, and an anti-biofilm agent for killing organisms, a water purifying agent for acting as a detergent, a sanitizer and a bactericide, a cleansing agent for acting as an astringent and an abradant in the removal of biofilm from contaminated surfaces and as a bactericide and fungicide, an anti oxidant and stabilizer agent, a scrubbing agent for acting as an abrasive and a cleanser for the removal of biofilm from contaminated surfaces, at least one pH adjuster for acidifying the disinfectant composition, and a diluent in the range of 35.0% to 50.0% by weight of the disinfectant composition. Barbeau et al., in PCT Patent Publication No. WO 00/27438, describe a composition for removing biofilm. This composition minimally comprises a detergent, a salt or a salt forming acid, and a bactericide.
A method and composition for suppressing or inhibiting the decomposition action of enzymes on hydrogen peroxide during bleaching of cellulose fibers with hydrogen peroxide in a way that microorganisms are not markedly affected is described in U.S. Pat. No. 5,885,412 to Paart et al. The composition contains hydroxylamine, thiocyanate salts, formic acid, ascorbic acid, or nitrites. It is suggested that the use of one or more of these substances suppresses or inhibits enzymes such as peroxidases and catalases from decomposing hydrogen peroxide, but does not affect microorganisms.
A more recent method for preventing biofilm growth has been to interfere with and prevent the chemical communication between cells in the biofilm, for example by utilizing antagonists of homoserine lactones. As in the Biblical story of the Tower of Babel, such approaches directly disrupt communication between the microorganisms contained in the biofilm, thus impeding the microorganisms' ability to coordinate their actions in order to replenish, expand and maintain the matrix, and ultimately leading to decomposition of the matrix. For example, Rycroft et al. in PCT Patent Publication no. WO 99/27786 describe compounds which may be used in the treatment or prevention of a bacterial infection in humans or in animals by controlling colonization of bacteria. The compounds may be employed to remove biofilms from surfaces. Davies et al. in PCT Patent Publication No. WO 98/58075 describe a method to control the formation, persistence and dispersion of microbial biofilms by taking advantage of the natural process of cell-cell communication in bacteria. As with treatment by extracellular enzymes, treatment of biofilms in industrial water using antagonists of homoserine lactones may be too specific, may yield varying results in different sites, or may not be cost-effective.
The present invention seeks to provide a method for controlling the development of biofilms. The present invention is based on the surprising observation that the biocides of the inventor's own U.S. Pat. Nos. 5,976,386 and 6,132,628, the contents of both of which are incorporated herein by reference, unexpectedly control biofilm development, at a feed rate and according to a feeding regime which are insufficient to cause significant killing of microorganisms embedded in the biofilms. The unexpectedly low feed rate and feed regime may be used to maintain biofilm-free surfaces, to remove existing biofilms and to limit the production of enzymes, including peroxide degrading enzymes such as catalases, peroxidases and dehydrogenases and starch-degrading enzymes such as amylases, which may otherwise be formed by the microorganisms embedded in biofilms. Furthermore, the present invention enables industrial operations involving process waters, such as paper bleaching or de-inking plants, to operate more efficiently, for example by reducing the amount of peroxide required during bleaching or de-inking, by reducing the frequency of boil-out, i.e. cleaning the papermaking machinery with hot, caustic water, and by reducing down-time due to boil-out and other cleaning operations. The present invention also enables optimization of industrial processes which utilize water, including the wet-end chemistry of industrial paper-making processes, by controlling the development of biofilms on the surfaces of fibers, suspended particles and additives. It has been recognized by the present inventor that the growth of biofilms on the surfaces of fibers and suspended particles can interfere with the binding of such fibers or particles, resulting in defects or reduced quality in the resulting paper.